Balancing of airfoils, such as the rotor blades of model helicopters, has been effected by one of two methods.
In the first method, two airfoils are mounted, in the normal fashion for ultimate use, by attaching their roots in-line on a hub from which protrude, orthogonally to the airfoils's major axis, two flybars. The flybars are placed horizontally on top of two parallel knife edges so the whole assembly is free to rotate around the flybars' axis. Mass is added to the airfoil rising above the horizontal until there is no perceived rotational movement. Careful use of this scheme results in each airfoil having very nearly the same rotational moment (the product of the distance to the center of gravity from the center of the hub times the mass times the local acceleration of gravity). A simple equation will show the significance of this scheme. Let A.sub.1 and A.sub.2 be the distance from the center of the hub to each center of gravity. Let M.sub.1 and M.sub.2 be the total mass of each airfoil, and let g be the local value of the acceleration of gravity. The moment of the first airfoil is A.sub.1 .times.M.sub.1 .times.g and must be equal to the moment of the second airfoil A.sub.2 .times.M.sub.2 .times.g because there is no rotation. However, this equation shows no requirement that M.sub.1 =M.sub.2 nor that A.sub.1 =A.sub.2 and thus this scheme cannot insure that the distribution of mass along each airfoil is such as to place the longitudinal center of gravity of each airfoil the same distance from the center of the hub nor can it insure that the airfoils have the same mass. If a separate adjustment is made to make M.sub.1 =M.sub.2, then the above equation shows that A.sub.1 will equal A.sub.2. This requires removing the airfoils from the hub, making them equal in mass, remounting them on the hub and iterating the above process. The iterations are so inconvenient to perform, and result in so much wear of the airfoils, that many do not bother to perform them.
An additional problem is that the flybars tend to deform or, to bow in use under the force of the assembly, and thus reduce the sensitivity to the detection of unequal moment.
The second method requires screwing the airfoils to a fixture, having a central pin around which balancing takes place, that functionally replaces the hub of the first method.
The new airfoil balancer overcomes the above limitations because, using only one device and without damage to the airfoils, one may conveniently and reliably adjust the mass and center of gravity of equal length airfoils to be equal. The invention accomplishes this by providing a convenient and non-destructive way to make both the root-to-root and tip-to-tip moments equal. Once again, an equation will help. Let L equal the length of each airfoil from root to tip. Let A.sub.1 and A.sub.2 be the root to center of gravity distances, and let M.sub.1 and M.sub.2 be the masses. When the root-to-root moments are equal: A.sub.1 .times.M.sub.1 .times.g=A.sub.2 .times.M.sub.2 .times.g. When the tip-to-tip moments are equal: (L-A.sub.1).times.M.sub.1 .times.g=(L-A.sub.2).times.M.sub.2 .times.g. Thus: M.sub.1 =M.sub.2 and A.sub.1 =A.sub.2.
The new airfoil balancer also provides increased sensitivity to differences in moment produced by low mass airfoils such as the tail rotor blades of a model helicopter. It accomplishes this by providing two reference positions an equal distance from the axis of rotation. Thus, using these offset reference positions, the moment arms are significantly longer than they would otherwise be and result in greater sensitivity.